In the related art, there is known a vehicle seat in which a seat frame is formed in a closed sectional shape with a constant lateral sectional shape by extrusion-molding an aluminum material (see, for example, JP-A-2015-101286). In the seat frame of the closed sectional shape, reduction holes which make an opened section in plural positions are formed to optimize a balance between structure strength and weight.
In the configuration in which a portion of the seat frame with the closed sectional shape has a reduction hole similarly to the conventional structure, a section modulus is rapidly changed so that a stress concentration may easily occur.